PS6'1: The H2O Cycle

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PS6.1 The H2O Cycle

• NASA estimated that the world's reservoirs store
  5,000 km3 of water. How does this compare to the
  amount of water stored in natural freshwater
  lakes?
• What effect does human storage of freshwater
  have on sea level?
• Sea level is 0.5 lower than it would have been.

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PS6.1 The H2O Cycle

• Reservoirs increase evaporation and reduce
  runoff. NASA researchers estimate that this has
  decreased total runoff by about 600 km3/y. What
  effect does this have on sea level?
• This is a subtle question. If evaporation
  increases, precipitation must increase, too. The
  increased precipitation onto the ocean will
  compensate exactly for the decreased runoff.
  Because the residence time of water in the
  atmosphere is short, this new equilibrium is
  established quickly.

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PS6.1 The H2O Cycle
Note that PO EO R. If EO remains constant
(why would it change?), then any decrease in R
due to an increase in EL must be compensated by
an equal increase in PO.
Atmosphere
EO456
PO410
EL62
PL108
R 46
Land
Ocean
Atmosphere
EO456
PO411
EL63
PL108
R 45
Land
Ocean
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PS6.1 The H2O Cycle

• What is the average residence time of water in
  reservoirs with respect to evaporation?

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PS6.1 The H2O Cycle

• Humans also pump groundwater most evaporates or
  runs off into streams. Suppose that 25 of all
  water used by humans is from groundwater, and
  that 25 of this groundwater becomes runoff. What
  effect does this have on sea level?

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PS6.2 The C Cycle

• Current worldwide emissions of CO2 from
  fossil-fuel burning currently total about 6.6
  GtC/y. If all this CO2 remained in the
  atmosphere, at what rate would CO2 concentrations
  increase, in ppmv/y?
• .

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PS6.2 The C Cycle

• The preindustrial concentration was 275 ppmv. If
  all the CO2 released by fossil-fuel burningabout
  300 GtCremained in the atmosphere, what would
  the atmospheric concentration of CO2 be today?
• .

Compare to actual 375 ppmv
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PS6.3 The N Cycle

• Total crop harvest ? 3 Gt/y. Assuming dry biomass
  is H2960O1480C1480N160P18S10, how much N is
  removed?
• World production of N fertilizer is 90 MtN/y.
  What proportion of N requirement could
  manufactured fertilizers be supplying?

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PS6.3 The N Cycle

• If half of N washes off and enriches growth of
  natural vegetation, how many billion tons of
  carbon (GtC) would be sequestered each year?
• What fraction of annual carbon emissions (6.6
  GtC/y) does this correspond to?